GaN-based crystals are used for short-wavelength light emitting devices. Usually, a GaN-based crystal is epitaxially grown on a sapphire substrate. Instead of this method, it is desired to epitaxially grow a GaN-based crystal having a low dislocation density on a Si substrate. GaN-based crystals are available in two different crystal structures, namely, a wurtzite-type structure having a superb piezoelectric characteristic and a Zinc-blende type structure that has almost no piezoelectric characteristic and has a high capability in information transfer or information processing. Generally, a wurtzite-type structure crystal is stably grown at high temperature. Meanwhile, a Zinc-blende type structure crystal, which is metastable, promotes a good recombination and therefore, when being used for a short-wavelength light emitting device, significantly increases the light emission efficiency of the device. For this reason, a technology for growing a Zinc-blende type structure crystal at a low dislocation density is strongly desired. In addition, a GaN-based crystal of a Zinc-blende type structure having a low dislocation density, when obtained, realizes, for example, a composite device of a short-wavelength light emitting device and a semiconductor integrated circuit. Such a device will be developed into a variety of useful devices in the future.
The present inventor reported an example of forming a GaN crystal directly on a BP crystal film that is formed on a Si substrate (Japanese Laid-Open Patent Publication No. 2003-229601). However, in actuality, phenomena that the crystal has a roughened surface, is partially delaminated, is cracked, or the like occasionally occur. There is another example in which a GaN-based crystal is formed on a BP crystal film that is formed on a Si substrate with an intermediate layer being located between the GaN-based crystal and the BP crystal film (Japanese Laid-Open Patent Publication No. 2000-235956). In this case, a Zinc-blende type GaN-based crystal is not formed.